The relation of the structure of the retina to factors influencing visual acuity is examined in some detail. Dimensions of the foveal cones have been redetermined in a series of measurements on histological materials from several sources. In no case has center to center distance between the most central cones in adult human eyes been found less than 2.0μ nor greater than 2.3μ, these being values for the living eye, due allowance being made for shrinkage of the preparations. Photomicrographs of the central foveal mosaic are shown to illustrate the rapid increase in center to center distance between cones a short distance from the small central fovea. These results are in general agreement with those of Polyak. This cone mosaic is in good accord with the observed effect on resolution acuity of the particular location of the image within the fovea. The central cones are of such size that the optical system of the eye at optimum pupil concentrates approximately three-quarters of the energy of a star image into a circle containing three cones, but some additional optical mechanism must exist in the retina to prevent loss of this image quality by spread of rays after the focus, but within the retinal depth represented by the length of the cones. Optical processes occurring within the cones are considered in detail. It is shown that not only is the Stiles and Crawford effect explained quantitatively by these processes, but also that the spread of rays beyond the focus is prevented, so that the retinal mosaic can behave as though the light receptors were concentrated in the focal plane. This same process also makes possible greatly increased light absorption per unit weight of cone pigment. The structure of the retina is thus consistent, numerically, with resolution acuity at high illumination but does not account for observed changes in acuity as retinal illumination is reduced. Ballistic stimulation of 10 microsecond duration eliminates effect of eye movements and serves to test both the Hecht theory of change of acuity with luminance, and the alternate hypothesis that very few quanta are absorbed per cone at threshold, statistical variation in this number accounting for the acuity change. The result of these experiments indicates that neither theory can be accepted.
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